So over here we have this wonderful crystal radio.

Now.

We covered a lot of the physics, but one thing we didn't really focus on because there's an awful lot.

Even though this is just a few components, this is.

There's an incredible amount of physics and electronics going on in this.

The core of this, as well as the day old, is what's called a tune circuit.

So when I had just this so nothing.

Now what I'm doing is I'm changing the capacitance on.

That's changing what's called the resonant frequency of this circuit frequency at which it's most sensitive.

Resonance is absolutely crucial across all physics under all engineering, its 1st 2nd 3rd year undergraduate postgraduates right across the board.

It crops up time and time again from everything from pushing somebody on a swing or a mass on a spring.

I like this monkey bumping up and down a particular writ at its natural or resonant frequency, and that set in this case by the stiffness of the spring on the mass of the monkey.

It's a very simple equation.

Similarly, here what we're doing is we're choosing a particular frequency out of Iran's of frequencies.

This particular station broadcast that I think something like, I've got this wrong absolute radio.

I apologize, but it's something like 1.2 megahertz, so 1.2 million times a second, the radio waves moving back and forth.

But this, with this wire by itself, isn't just picking up that signal.

It's picking up all the other signals as well.

So what you need to do is to narrow in on get that frequency, which is what was happening earlier.

Wonderfully, we've got a mechanical.

We can see this idea of residents really straightforward.

This is a very simple set up.

We've got a bar of ruler basically, and it also has its own natural frequency.

We're actually gonna drive it mechanically.

There's a solid night here and there's a magnet.

So we're just driving it back and forth with a magnet and to monitor the motion.

We could look at this, but we're gonna do something even nicer, which is we've got a laser pointer.

That's quite neat, because what we have is something that's going to tell us just how much this is vibrating.

So what's happening is the lady's going down and there's a little angel at the moment.

You can see it's bobbing up and down, but not very much know so worked for her.

It's 4.4 hertz.

Brady.

So what I'm gonna do is I'm gonna turn the frequency up notice.

I'm not driving it the amplitude stairs to see him.

All I'm changing is the frequency.

This is really important.

Okay, so it's going up going up, and, of course, the frequencies going up.

So expansion around a little bit more 12 hurts.

14 hearts, 15 hurts.

Okay, so we're getting some, but not much.

16.

17 18 now, a little bit more, but not much.

Still, that's not much more than when we wear it for Hertz.

That's off residence, right?

So we haven't hit the natural frequency.

Remember, we had the monkey on the real hell.

It's so what I'm gonna do is find the natural frequency.

See it building up.

Oh, yeah.

Right.

So we're at two resonant frequency right now.

If we go, the other side of that happened.

Why's that thing suddenly gone?

Yeah, I'm going crazy.

because what you've done is exactly what the radio circuit is.

You're driving it with the right frequency.

It's got its own natural frequency just as the monkey on this thing or any mass on a spring has its own natural frequency.

This has its own natural frequency on.

Now you're driving it at that natural frequency.

And what is that frequency Sonar frequencies about 25 hurts.

The difference is, is when you're aware from that frequency you don't get more genital transfer, so it's still vibrating.

But when you hit that residency, resonant frequency, that's where you got a lot of energy.

Transfer into it on it actually access filter.

Because if we go above that when I was 33 hurts for now with the other side and stay enough and you can see that it's reduced almost to what itwas when we already 18 hurts.

So you're saying when you put in the correct natural resonance frequency energy transfer her.

Does that mean it's kind of like t use a term you probably won't like?

It's almost like the stars have aligned, and this is like, yeah, the way all my structure works in my length and my what I made off.

This is just the perfect thing for everything.

Precisely, you can do this experiment without driving it.

This ruler.

Its resonant frequency depends on its lens.

So if I make this long, relatively low, resonant frequency for America's short hi arrest infrequent, you can even hear the note changing resident frequencies going down.

And it's so if I took this now, when I drove it at that frequency, I get a lot of energy transfer and you'll get this big response.

That's what's happening here, but it gets better so we can draw a parallel between what's here on what's here.

So here we've got a mass Andi, we've got, ah, some friction and that's, you know, freaks.

And due to the broad itself, but also with the air on, we're driving it with this on.

What we also have is some stiffness, because the rules got some stiffness.

Over here.

We've got inducted ce capacitance.

It's being driven by this by the radio waves on what's absolutely remarkable is that the inductive is here.

If we write down the equations, I'm not going to do that.

Don't worry, Brady, but if you write down the equations.

What we find is that we can do a 1 to 1 mopping conduct insta mass capacitance to spring constant to stiffness.

Also, in terms of the driving force.

In this case, it's a mechanical driving force.

In this case, it's an electrical driving force on the equation.

The differential equation you write down is identical on the solutions are identical.

So that's where you see residents here, and you also see residents here are different frequencies, but it's the same phenomenon.

So that moment when you got that bar to jump up and down like crazy, there is the same as the moment when you got absolute radio perfect black or dark red, the highest points gonna be white and then scale it.

Or we can do what we're doing here and we can listen to it.